1. Field of the Invention
This invention relates to a thrust ball bearing, and more particularly to a thrust ball bearing of great load capacity and low friction which requires high-speed rotation or a thrust ball bearing having the same degree of load capacity as the ordinary bearing and yet used for low friction or high-speed rotation and long in life and easy and inexpensive to manufacture.
2. Description of the Prior Art
With the recent tendency toward higher speed of the support bearings in machine tool main spindles, jet engines, gas turbines, high pressure pumps, traction drive infinitely variable transmissions or ball screws, the friction loss due to the spin of balls during the rotation of the bearing is great in the conventional thrust ball bearings and, if the centrifugal force applied to the balls with the high-speed rotation becomes greater, the spin loss doubles and the efficiency of the instruments is significantly reduced. For this reason, it is a matter of common knowledge to consider thrust ball bearings unusable for such applications. Accordingly, angular ball bearings or the like which are capable of effecting high-speed rotation have heretofore been used for these applications. However, thrust ball bearings are apparently more advantageous in respect of the thrust load capacity and friction torque during low-speed rotation and therefore, realization of thrust ball bearings of low friction capable of effecting high-speed rotation has been demanded.
U.S. Pat. No. 3,455,618 more or less meets such demand. The invention of this patent is such that, as shown in FIG. 1 of the accompanying drawings, the smooth track surfaces 24 and 26 of races 20 and 22 are conically disposed so that the cone center d is coincident with the center axis l of the bearing, and a control ring 30 is fitted to the circumscribed circle of each ball 28. Accordingly, as shown in FIG. 2 of the accompanying drawings, the contact ellipses A", B" and C" between the balls and each track surface are substantially circular small points and the surface pressure thereof is quite high. Although centrifugal force of the balls during high-speed rotation is irrelevant to thrust load, the influence of centrifugal force on the bearing is substantial; lubricant can hardly be supplied to the point of contact C between the ball 28 and the control ring 30. Moreover, the invention of this patent cannot be practically used for high load and high-speed rotation and can merely be used in the case of light load, medium speed and low friction torque experienced in clutch release, as described in the specification of the patent.
In contrast with this U.S. patent, in the conventional popular thrust ball bearing, as shown in FIG. 3 of the accompanying drawings, the cross section of the track grooves of races 1 and 2 is arcuate and therefore, the area of the contact ellipse between the track surface and the ball is great, and the thrust load capacity is maximum because the positions A and B of the load points of the balls 3 are set to .theta.=90.degree. relative to the plane of revolution of the balls, as shown. On the other hand, the tangential lines at the load points A and B are parallel to each other and do not intersect each other and therefore, great spin motion occurs between the balls and the groove. The friction loss resulting from such spin is great as compared with other factors such as elastic hysteresis and differential slippage. Furthermore, when the bearing rotates at a high speed, centrifugal force acts on the balls and the points A and B become deviated outwardly and therefore, the spin loss is further increased. Thus, it has been practically difficult to use such bearing during high-speed rotation.
It is known that this spin loss becomes theoretically null if the tangential lines at the load points of the balls and the grooves are made to intersect each other on the center axis of the bearing. Accordingly, if a member such as the control ring 30 of the aforementioned U.S. patent is fitted to the circumscribed diameter of the ball and the point of intersection between the tangential lines is linked with the center axis as described above with the balls placed inwardly of the track diameter, the spin loss between the track surface and the ball becomes null, but if a great thrust load which the track can withstand is applied to the track, a very great surface pressure is created at the point of contact between the ball and the control ring 30 as previously noted and an impression is created in the track surface of the control ring 30 and thus, such thrust ball bearing cannot be practically used.